This invention relates to a sustain driver that applies a sustaining voltage pulse to the electrodes of a plasma display panel (PDP), and in particular, relates to the control circuit for the sustain driver.
Plasma displays are display devices of the self-emission type, which use a light emission phenomenon caused by a discharge in gas. Plasma display panels (PDPs) are easy to upsize and slim down in contrast to other display devices, and furthermore, have advantages in flicker-free images, high contrasts, high-speed responses, and so on. Because of these advantages, plasma displays have become widespread in recent years, which are served as next-generation image display devices in place of CRTs (cathode-ray tubes).
A PDP comprises a basic structure with two substrates laminated. For example, in the structure of an AC type PDP, or in particular, the three-electrode surface-discharge type structure, a plurality of address electrodes are arranged on the rear substrate in the vertical direction of the panel, and a plurality of sustain and scan electrodes are alternately arranged on the front substrate in the horizontal direction of the panel. As another example, in the structure of a DC type PDP, or in particular, the pulse memory type structure, a plurality of anodes are arranged on the rear substrate in the vertical direction of the panel, and a plurality of cathodes are arranged on the front substrate in the horizontal direction of the panel. Discharge cells are placed at the intersections of the vertical and horizontal electrodes. A layer including phosphors is provided on the surfaces of the discharge cell. Gas fills the inside of the discharge cell.
In an AC type PDP, light emissions occur, for example, as follows. First, a high voltage pulse is applied between the scan and address electrodes. At that time, discharge in gas occurs in the discharge cell located at the intersection of those electrodes. Gas molecules in the discharge cell ionize to cations and electrons, which stick onto the surfaces of the discharge cell. Thus, wall charges accumulate on the surfaces of the discharge cell. Next, high voltage pulses (sustaining voltage pulses) are periodically applied to the sustain electrodes. On the other hand, the scan electrodes are maintained at, for example, approximately half the height of the peak of the sustaining voltage pulse. Thereby, an alternating voltage appears between the sustain and scan electrodes in each discharge cell. In a discharge cell accumulated the wall charges in advance, discharge in gas occurs due to the sum of the voltage induced by the wall charges and the sustaining voltage pulse. Gas molecules in the discharge cell ionize, and thereby, emit ultraviolet rays. The ultraviolet rays excite the phosphors on the surfaces of the discharge cell, and then, cause them to emit fluorescence. On the other hand, the gas molecules in the discharge cell ionize to cations and electrons, which accumulate on the surfaces of the discharge cell again. Accordingly, the gas discharge and fluorescence are repeated in the discharge cell at every reversal in polarity of the voltage between the sustain and scan electrodes. Thus, the discharge cells sustain the light emissions.
In a DC type PDP, light emissions occur as follows. First, the high voltage pulse is applied between the cathode and the anode. At that time, discharge in gas occurs in the discharge cell located at the intersection between those electrodes. Gas molecules in the discharge cell ionize to cations and electrons, which remain within the discharge cell, serving as priming particles. As a result, a breakdown voltage is reduced. Next, high voltage pulses (sustaining voltage pulses) are periodically applied to the cathode. At that time, discharge in gas occurs in the discharge cell in which the priming particles remain, since the breakdown voltage is lower than the peak of the sustaining voltage pulse. Gas molecules in the discharge cell ionize, and thereby, emit ultraviolet rays. The ultraviolet rays excite the phosphors on the surfaces of the discharge cell, and cause them to emit fluorescence. On the other hand, the gas molecules in the discharge cell ionize to cations and electrons, which remain again, serving as the priming particles. Accordingly, the gas discharge and fluorescence are repeated in the discharge cell at every application of the sustaining voltage pulse. Thus, the discharge cells maintain the light emissions.
A sustain driver is a device that applies sustaining voltage pulses to the electrodes of a PDP. For example, in an AC type PDP, the sustain driver is connected to the sustain electrodes. In a DC type PDP, the sustain driver is connected to the cathodes.
The sustaining voltage pulse is usually higher than 200 V. Transient potential fluctuations inside the device are further added to the voltage that the sustain driver should withstand. Reliable operation is required of the sustain driver under such high voltage conditions.
FIG. 8 is an equivalent circuit diagram that shows an example of a conventional sustain driver. See, for example, Published Japanese patent application Hei 4-230117 gazette. This sustain driver comprises a floating voltage generating circuit 30, a control circuit 100, and an output circuit 20.
The floating voltage generating circuit 30 controls each potential of four power supply terminals 2H, 2F, 2L, and 2G of the control circuit 100. Thereby, the high side power supply terminal 2H is maintained at a potential higher than the potential of the floating power supply terminal 2F, which is hereafter referred to as a floating voltage, by the voltage across the capacitor 33. Here, the voltage across the capacitor 33 is maintained at a constant value, for example, the voltage (for example, 15 V) of an internal constant-voltage source 31. The low side power supply terminal 2L is maintained at a constant potential, for example, a potential higher than the ground potential by the voltage (for example, 15 V) of the constant-voltage source 31. The low potential power supply terminal 2G is a ground terminal, for example, and is maintained at the ground potential.
The control circuit 100 receives two kinds of control signals, which are hereafter referred to as high and low side input signals, from the outside such as the main control section of the plasma display. The high side input signal is converted by the level shift circuit 4 and the high side circuit 5H into a control signal for a high side power MOSFET 22H inside the output circuit 20, which is hereafter referred to as a high side output signal. Here, the high side circuit 5H is generally a circuit with a MOSFET input, and operates on the voltage between the high side power supply terminal 2H and the floating power supply terminal 2F. The low side input signal is converted by the low side circuit 5L into a control signal for a low side power MOSFET 22L inside the output circuit 20, which is hereafter referred to as a low side output signal. Here, the low side circuit 5L operates on the voltage between the low side power supply terminal 2L and the low potential power supply terminal 2G.
In the output circuit 20, the two power MOSFETs 22H and 22L are connected in series between the high potential power supply terminal 21 and the ground terminal. Here, the high potential power supply terminal 21 is connected to an external constant-voltage source, and maintained at a predetermined high potential, for example, 200 V. The two power MOSFETs 22H and 22L are alternately turned on and off under the high and low side output signals, respectively. Thereby, the potential of the node of the MOSFETs or a voltage pulse output terminal 23 changes between two levels. The voltage pulse output terminal 23 is connected to the sustain electrodes of the PDP. Thus, the sustaining voltage pulses are applied to the sustain electrodes.
When the high side power MOSFET 22H is an n-channel MOSFET, for example, the floating power supply terminal 2F is connected to the node of the two power MOSFETs 22H and 22L, or the source of the high side power MOSFET 22H. Thereby, the level of the high side output signal with reference to the source of the high side power MOSFET 22H changes around the threshold value of the high side power MOSFET 22H, regardless of the turn-on or off of the high side power MOSFET 22H. In that case, the potential of the floating power supply terminal 2F, or the floating voltage changes between the ground potential (0 V) and the potential of the high potential power supply terminal 21 (for example, 200 V), in response to the turn-on and off of the two power MOSFETs 22H and 22L. In synchronism with the change, the high side power supply terminal 2H changes its potential. The range of the change is higher than the range of the floating voltage by a constant level, for example, 15-215 V.
During the period when the high side power MOSFET 22H is maintained in the ON state, the high side power supply terminal 2H is maintained at a potential higher than the potential of the high potential power supply terminal 21. When the high side input signal indicates the OFF state of the high side power MOSFET 22H, the transistor 4T inside the level shift circuit 4 is turned on. At that moment, the potential of the node of the transistor 4T and the high side circuit 5H, or an input terminal 5A of the high side circuit 5H abruptly drops from the neighborhood of the potential of the high potential power supply terminal 21 near to the ground potential. Thereby, very large and transient potential difference appears between the high side power supply terminal 2H and the input terminal 5A of the high side circuit 5H. The high side circuit 5H has generally a MOSFET input. The MOSFET input section 5B detects a change in the potential difference between the input terminal 5A of the high side circuit 5H and the high side power supply terminal 2H (or the floating power supply terminal 2F). If the potential difference, even transiently, exceeds any withstand level of the source-gate, drain-gate, and backgate-gate voltages of the MOSFETs included in the MOSFET input section 5B, the MOSFETs may malfunction. Furthermore, the MOSFETs may be at the risk of destruction. In addition, the malfunction of the high side circuit 5H leads the malfunction of the output circuit 20, and thus, spoils the reliability of the output circuit 20, and furthermore, increases the risk of the simultaneous turn-on of the two power MOSFET 2H and 2L. In that case, the two power MOSFET 2H and 2L may be destroyed by shoot-through current.
In the conventional control circuit 100, the anode and cathode of a Zener diode 70 are connected to the input terminal 5A of the high side circuit 5H and the high side power supply terminal 2H, respectively. The Zener diode 70 is turned on at the time when the potential difference between the high side power supply terminal 2H and the input terminal 5A of the high side circuit 5H reaches a constant breakdown voltage (Zener voltage). Thereby, the potential difference between the high side power supply terminal 2H and the input terminal 5A of the high side circuit 5H is clamped to the Zener voltage. Thus, the malfunction and destruction of the high side circuit 5H due to overvoltage are prevented. As a result, the high side circuit 5H operates reliably even if a high voltage of about 600 V, for example, is applied between the high side power supply terminal 2H and the input terminal 5A of the high side circuit 5H. In the conventional sustain driver as shown in FIG. 8, as described above, the Zener diode 70 connected between the high side power supply terminal 2H and the input terminal 5A of the high side circuit 5H protects the high side circuit 5H from overvoltage. The higher reliability this overvoltage protection has, the higher reliability the sustain driver has.
When the control circuit 100 of the sustain driver is configured as a single integrated circuit, for example, the base-emitter junction of an npn bipolar transistor is used as the above-described Zener diode 70. At the turn-on of the transistor 4T inside the level shift circuit 4, the reverse current flows through the Zener diode 70, or the above-described base-emitter junction. The voltage across the Zener diode 70 includes, in addition to the Zener voltage, the voltage drop due to the above-described reverse current and the resistance of the base-emitter junction against the reverse bias. For the overvoltage protection, it is desirable that the voltage drop across the Zener diode 70 is maintained sufficiently lower than the Zener voltage regardless of the amount of the reverse current, since the voltage across the Zener diode 70 is maintained substantially equal to the Zener voltage regardless of the amount of the reverse current. Accordingly, the above-described resistance of the Zener diode 70 has to be reduced for the further improvement in reliability of the overvoltage protection. However, a very larger area has to be allocated to the Zener diode 70 with the above-described resistance lower, in comparison with the areas of the other circuit elements, since the above-described resistance depends on the area of the PN junction inside the Zener diode 70. Thus, the maintenance of the high reliability of the overvoltage protection prevents the further higher integration of the control circuit 100. As a result, further miniaturization of the sustain driver and its resulting further reduction of the manufactures' costs are difficult.